1. Field of the Invention
The present invention relates to a method of improving the temperature profile of a furnace, and especially the temperature profile of a furnace for reheating or annealing products, more particularly steel products such as slabs or billets or flat products.
2. Related Art
Reheat and/or heat-soak furnaces are generally used in the steel industry, especially in continuous casting cycles in which liquid metal is cast to form intermediate products which, before they pass into the rolling mill at the end of the continuous casting line, pass through at least one furnace called a reheat furnace in which the intermediate product is brought to or soaked at a suitable temperature, which is as uniform as possible, so as to be subsequently rolled under optimum conditions.
A reheat furnace of this type and its method of use is described, for example, in EP-A-0 370 916.
In a reheat furnace, there is generally at least one zone equipped with heating means, for example burners, so as to provide the energy necessary for reheating or heat-soaking the metallurgical products. Three main zones are often distinguished in a furnace, especially a reheat furnace, these being characterized by different heat transfer modes:                the actual heating or firing zone where combustion develops (with burners, plasmas, auxiliary heating, etc.), in which the ambient temperature or the temperature of the product is controlled and if necessary regulated. This zone is characterized by the fact that the heat is produced in situ, this produced heat being a direct means of controlling and/or regulating the temperature in this zone;        the flue gas exhaust zone, also called the dead zone, in which the flue gases or the atmosphere coming from the heating zone lose some of their energy to the steel product entering the furnace. This zone helps to optimize the thermal efficiency of the process; and        the so-called heat recovery zone, comprising a heat recovery system that allows some of the energy still present in the flue gases or in the atmosphere leaving the furnace to be transferred to the oxidizer needed for combustion (oxidizer preheating). By recycling, this energy is reintroduced into the furnace, which optimizes even further the overall energy budget.        
External limitations aside, current regulation systems make it possible to operate the heating zone and the recuperator at their nominal running conditions by simply controlling various direct parameters (burner power, auxiliary energy, heat exchange in the recuperator, etc.) to regulate these zones.
In contrast, the dead zone of a furnace is not regulated as the heat exchange is completely dependent on the operating parameters of the heating zone and, where relevant, of the recuperator. In particular, the thermal profiles in said zone are not optimized as they depend on the materials flow conditions (the materials being the flue gases and the metallurgical products) in the furnace and on their temperature conditions.
The sole constraint that is generally imposed by the furnace operator is the “exhaustion” of the flue gases (optimum heat exchange between the flue gases and the products or the furnace) in this zone so as to comply with the limitation on the temperature at which the flue gases leave this zone so as not to damage the flue gas recovery system and limit the heat losses by the flue gases if they are not recovered.
In the construction of the furnace, this zone is therefore designed to provide sufficient cooling of the flue gases output by the heating zone.
FIG. 1 explains better the operation of a reheat furnace of known type and the problems to be solved in order to improve its operation.
In FIG. 1, the reheat furnace 1 is shown schematically with the metallurgical products 5 moving forward (thanks to a system of beams 14 and drive means, these not being shown in FIG. 1) from right to left, the direction of advance of these products being indicated by the arrows 7.
The furnace 1 comprises here a heating zone 2 in which the temperature varies between 1200° C. and 1400° C., this zone 2 being fitted with burners and having one or more regulating zones. The burners are not shown in this figure. Only the hot air circuit is schematically indicated (13). The furnace also includes a dead zone 3, which is generally the preferential flue gas path, in which the flue gas temperature is generally around 900° C. to 1100° C., which is sufficient for effective preheating of the steel products by heat exchange and finally a underheated zone 4 that forms part of the dead zone 3, generally located near the point of entry of the steel products and generally above them (in particular if the flue gas recovery line 8 lies below the point of entry of the steel products) and the temperature of which zone varies between 600° C. and 900° C.—this temperature is generally too low for effective preheating of the steel products.
The flue gases are used to preheat the air (oxidizer), coming from the oxidizer generator 11 via the line 10, in the recuperator 9 from which the preheated oxidizer exits along the line 12 that feeds the burners 13 with oxidizer (the fuel lines for the burners are not shown in FIG. 1).
In such an operating mode, the dead zone 3 is heated little throughout the duration of production and there exists in fact the possibility of having, at least for part of the production time, a higher temperature in this zone so as to preheat the steel product better.
However, this temperature increase in the dead zone must not result in a corresponding increase in the temperature of the flue gases exiting the furnace. This is because, although the temperature of the dead zone (and therefore of the flue gases) can be increased by about 200° C. for example, the problem that arises is that it is undesirable for these flue gases to exit the furnace at a temperature 200° C. higher than their usual exit temperature. The problem is that if there is no recuperator at the furnace exit, then since the flue gas temperature is 200° C. higher, all the corresponding thermal energy is lost and the thermal (and therefore financial) budget of the furnace becomes unacceptable. Likewise, if there is a recuperator (as indicated in FIG. 1), which usually works at a temperature close to its maximum temperature (in other words, the temperature of the flue gases entering the recuperator is close to the maximum temperature that the recuperator can withstand without being damaged), it is not possible to supply it with flue gases whose temperature has been increased by 200° C. Consequently, a person skilled in the art is faced with the problem of substantially increasing the temperature of the dead zone of the furnace, and especially its crown temperature, without a corresponding substantial increase in the temperature of the flue gases exiting the furnace.